In recent years, electronic equipment has been becoming increasingly smaller in size and lighter in weight, and there has been a steady increase in demand for a highly enhanced energy density battery as a power source. Zn, Mg, Cd, Al, Ca, Fe or the like as a negative active material for such a battery has high energy density and power density per unit mass, and moreover, there are advantages that those are inexpensive and excellent in safety.
However, in a negative electrode containing Zn (zinc) or the like as an active material, dendrites of zinc or the like may grow at the time of charging since the solubility of zinc is high and then the dendrites may penetrate through a separator. This may result in a short circuit, and this may cause the charge-discharge cycle life to be shortened (see, for example, JP 6-275310 A).
In JP 6-275310 A, there has been proposed “a sealed alkaline zinc storage battery in which an electrode groups prepared by layering a zinc negative electrode composed mainly of zinc oxide and metallic zinc, a positive electrode, and a liquid holding layer and a separator which are interposed between the zinc negative electrode and the positive electrode, wherein the liquid holding layer and the separator are impregnated with an electrolyte solution, and wherein polyethyleneimine having a molecular weight less than or equal to 10000 is added into the electrode group.” (See claim 1.) Also it has been shown that “since crystals of metallic zinc attempting to grow into dendrites are surrounded by a polyethyleneimine having a molecular eight less than or equal to 10000, the growth of the crystals can be suppressed.” (See paragraph [0008].)
Moreover, it has been described that “As a polyethyleneimine to be added into the electrode group of the above-mentioned sealed nickel-zinc storage battery, a polyethyleneimine with a structure in which a primary amino nitrogen atom, a secondary amino nitrogen atom or a tertiary amino nitrogen atom is contained is preferred.” (See paragraph [0010].) Also it has been described that “even when any polyethyleneimine having a certain molecular weight listed in Table 2 is added to the electrolyte solution, sufficient effects are not attained in the point of enhancing the number of cycles because the solubility is small, but by adding the polyethyleneimine to the zinc negative electrode or the liquid holding layer, the number of cycles can be greatly enhanced.” (See paragraph [0019].)
In JP 2006-286485 A, there has been described “an alkaline battery allowing a negative electrode gel including zinc or a zinc alloy as a negative active material to face a positive active material with a separator interposed between the negative electrode gel and the positive active material, wherein a chelating reagent is added to the negative electrode gel.” (See claim 1.) Also it has been described that “an object thereof is to provide an alkaline battery capable of making the thickness of a separator thin to increase the amount of power generating material, effectively suppressing crystals of zinc oxide from being generated/growing due to discharging and then from penetrating through the separator, and thereby, comprehensively enhancing the discharge performance.” (See paragraph [0008].) Moreover, it has been shown that ethanolamine, oxalic acid, ethylenediamine, ethylenediaminetetraacetic acid, glycine, iminodiacetic acid, nitrilotriacetic acid or cyclohexanediaminetetraacetic acid as a chelating reagent was added to the negative electrode gel. (See Table 1, paragraphs [0033] to [0044].)
Furthermore, there have been pointed out the generation of hydrogen gas caused by a side reaction, the dendrite generated at the time of precipitation of zinc and the shape change as causes for a shortened life of the charge-discharge cycle in a zinc/air battery prepared with an aqueous electrolyte solution (see, JP 2013-84349 A). In JP 2013-84349 A, there has been described “a electrolyte solution for an alkaline battery including at least an organic substance having two or more carbon atoms and having one or more hydroxyl groups in its molecule.” (See claim 1.) It has been described that “an object of the present invention is to provide an electrolyte solution for an alkaline battery and an alkaline battery capable of suppressing the generation of hydrogen gas caused by a side reaction, the dendrite generated at the time of precipitation of zinc and the shape change of zinc to attain a prolonged charge-discharge cycle and excellent charge-discharge efficiency.” (See paragraph [0007].) Also it has been described that “by allowing the electrolyte solution to have such a configuration, when applied to an alkaline secondary battery such as an air-zinc secondary battery and a nickel-zinc secondary battery, it is possible to suppress the generation of hydrogen gas caused by a side reaction, the dendrite generated at the time of precipitation of zinc and the shape change of zinc. As a result, it is possible to attain a prolonged charge-discharge cycle and excellent charge-discharge efficiency.” (See paragraph [0024].)
In JP 58-184274 A, there has been described “a secondary battery including cadmium or zinc as a cathode active material, wherein an additive reagent which reacts with cadmium or zinc to form a complex ion or a chelate ion is added.” (See claim 1.) Also it has been described that “ . . . in a sealed secondary battery including cadmium or zinc as a cathode active material, an object thereof is to provide a secondary battery capable of surely preventing an increase in battery inner pressure and becoming a completely sealed rechargeable battery” (See page 2, upper right column, 7th to 11th lines.) Moreover, there has been shown ethylenediamine as an additive reagent which forms a complex ion or a chelate ion. (See same page, lower left column, 3rd to 6th lines.) It has been described that “in the case where the additive reagent is added to an electrolyte solution, the addition amount thereof is preferably 0.05 to 2 wt % or so.” (See same page, lower right column, 7th to 9th lines.) In examples, it has been shown that hexamethylenediamine in an amount of 0.1% is added to the electrolyte solution. (See page 3, upper left column, 4th to 2nd lines from the bottom.)
As described in JP 6-275310 A, JP 2006-286485 A, and JP 2013-84349 A mentioned above, in an electrode where zinc or the like is used as an active material, the dendrite formation and the shape change (a phenomenon in which the active material is gathered in the vicinity of the center of the electrode plane) occur during charging/discharging, and it is considered that this is a factor of deteriorating discharge performance and cycle characteristics, but even when an organic substance having an imino group, a carboxyl group, a hydroxyl group or the like, and the like are added to the electrolyte solution, the cycle characteristics have not been sufficiently improved.
In JP 2006-286485 A, it has been described that ethanolamine, ethylenediamine or the like is added, but this invention discloses that the above-mentioned amine is added to the negative electrode gel and is not aimed at enhancing the cycle characteristics.
In JP 58-184274 A, it has been described that ethylenediamine, hexamethylenediamine or the like is added to the electrolyte solution, but the addition amount is small and a technique for enhancing the cycle characteristics of a battery prepared a negative electrode containing cadmium or zinc as an active material has not been shown.